Slope compensation method and circuit for a peak current control mode power converter circuit

ABSTRACT

A slope compensation method and circuit for a peak current control mode power converter circuit is provided. Since the power converter circuit has a synchronous signal of a driven signal of enabling the first primary switch and the second primary switch, a triangular wave signal is generated. The driven signals of the first and second primary switches determine the ramp up time of the triangular wave signal. The triangular wave signal is added to one of the output DC voltage feedback signal of the corresponding power converter circuit that are used to compare with a current peak value of the voltage feedback signals. Therefore, a high level triangular wave DC voltage feedback signal that is higher than the DC voltage feedback signal is formed, and the switching noises do not effect comparing result of a PWM controller of the power converter circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a peak current mode control powerconverter circuit, and more particularly a slope compensation method andcircuit used to the peak current control mode power converter circuitthat restrains switching noises when the peak current control mode powerconverter circuit is operating.

2. Description of the Related Art

A conventional DC (direct current) to DC power converter converts a DCpower source into one or multiple DC power sources of differentvoltages, so as to output the DC power sources to corresponding circuitunits on a circuit board or corresponding electronic devices.

The DC to DC power converter has different control methods. Onecontrolled method is a peak current. The peak current control mode powerconverter circuit has an advantage of simplified circuits, but has adrawback of poor noise control ability. With reference to FIG. 8, aconventional DC to DC converter circuit 50 with a peak currentcontrolled mode has a first forward converter 51, a second forwardconverter 52, and a pulse width modulation (PWM) controller 53.

The first forward converter 51 and the second forward converter 52 haveat least two sets of high frequency transformers T3 and T4 and twoprimary switches Q1 and Q2. Primary sides of the high frequencytransformers T3 and T4 are coupled to the corresponding primary switchesQ1 and Q2. The primary switches Q1 and Q2 are open or closed todetermine whether the primary sides generate currents or not. On theother hand, secondary sides of the high frequency transformers T3 and T4are output terminals of the DC to DC converter circuit 50 with the peakcurrent.

The PWM controller 53 has a first PWM output terminal OUT1 and a secondPWM output terminal OUT2, a first output voltage feedback terminal COMP1and a second output voltage feedback terminal COMP2, and a first currentfeedback input terminal CS1 and a second current feedback input terminalCS2. The two PWM output terminals OUT1 and OUT2 are coupled to controlterminals of the primary switches Q1 and Q2 of the first forwardconverter 51 and the second forward converter 52.

The two output voltage feedback terminals COMP1 and COMP2 are coupled tocorresponding DC output terminals of the first forward converter 51 andthe second forward converter 52, so as to acquire two correspondingvoltage feedback signals V_(error1) and V_(error2) of the two DC outputterminals. Moreover, the current feedback input terminals CS1 and CS2acquire current feedback signals of the primary sides of thecorresponding high frequency transformers T3 and T4.

The DC to DC converter circuit 50 modulates a pulse width of each outputsignal by a peak current mode control method. With further reference toFIG. 9, when the two primary switches Q1 and Q2 are alternatively drivenby a pulse width signal of 50% duty cycle, an optimal waveform ofcurrent feedback signals of measured currents of the two currentfeedback input terminals CS1 and CS2 is shown. When the first primaryswitch Q1 is conductive for a 50% duty cycle time period, acorresponding voltage value V_(CS1) of a feedback current is larger thana voltage value V_(COMP1) of a first voltage feedback signal at an inputterminal COMP1.

At this moment, the PWM controller 53 turns off the first primary switchQ1 and also simultaneously controls the second primary switch Q2 to beconductive. With the same manner, when the second primary switch Q2 isconductive for a 50% duty cycle time period, the PWM controller 53compares a corresponding voltage value V_(CS2) of the second currentfeedback input terminal CS2 with a target value V_(COMP2) of a secondvoltage feedback signal at an input terminal COMP2. When thecorresponding voltage value V_(CS2) of a feedback current is larger thanthe voltage value V_(COMP2) of the second voltage feedback signal, thePWM controller 53 turns off the second primary switch Q2 and alsosimultaneously controls the first primary switch Q1.

Therefore, the PWM controller 53 detects the corresponding voltagevalues Vcs1 and Vcs2 at the first and second current feedback inputterminal CS1 and CS2 in accordance with the current of the primary sidesof the high frequency transformers T3 and T4, and then to compare withthe voltage values Vcomp1 and Vcomp2 of the voltage feedback signals atthe two input terminal COMP1 and COMP2 respectively coupled to thesecondary sides of the high frequency transformers T3 and T4. Once thevoltage peak value of the voltage value Vcs1 or the voltage value Vcs2is larger than the feedback voltage value, the PWM controller 53alternatively changes on and off statuses of the two primary switches Q1and Q2.

However, when the two primary switches Q1 and Q2 are respectively drivenby a pulse width signal of larger than 50% duty cycle, the peak currentcontrol mode method is inferior to the aforesaid example. With furtherreference to FIG. 10, the two primary switches Q1 and Q2 arealternatively driven by a pulse width signal of 55% duty cycle as atarget value. When only the first primary switch Q1 is driven to beconductive at t₁₀, the primary switch Q1 should be turned off when atime point t₁₂ reaches the target value in theory. However, a time pointt₂₀ is ahead of the time point of t₁₂ of the target value, so that thesecond 9 primary switch Q2 is conductive. Since the second primaryswitch Q2 is driven to be conductive at the time point t₂₀, a currentwaveform of the first current feedback input terminal CS1 produces asurge at a time point t₁₁.

The surge makes the PWM controller 53 determine that a correspondingvoltage peak value V_(CS1) of a first current feedback signal is largerthan a voltage value V_(COMP1) of a secondary side feedback voltagesignal. Hence the primary switch Q1 is turned off at a time point t₁₁.The optimal waveform is shown as a marked dotted line, and the primaryswitch Q1 is turned off earlier before the target value reaches. Withfurther reference to FIG. 11, an output DC power of the first highfrequency transformer T3 and the second high frequency transformer T4can not avoid generating oscillation.

Similarly, when the second primary switch Q2 is conductive for 55% dutycycle t₂₀ to t₂₁₁, the primary switch Q1 is driven to be conductiveagain by the PWM controller 53 at a time point t₁₃. At the time pointt₁₃ the coupling effect is generated, so as to influence the primaryside current of the second conductive high frequency transformer T4 toproduce a surge. Hence a corresponding voltage peak value V_(CS2) of asecond current feedback signal is larger than a voltage value V_(COMP2)of a second voltage feedback signal at a time point t₂₁ before the timepoint t₂₁₁ of the target value, so as to turn off the second primaryswitch Q2. In comparison with the optimal waveform as the dotted line,the second primary switch Q2 is turned off earlier. Nevertheless, the DCpower oscillation is still generated.

Therefore, it can be clearly understood that when the PWM signals of thetwo primary switches Q1 and Q2 are larger than 50% duty cycle, operationperiods of the first forward converter 51 and the second forwardconverter 52 become overlapped. At this moment, the turned off primaryswitch generates a noise and the noise is coupling to the other currentwaveform, so as to influence the peak current and further result in anoutput oscillation phenomenon. Thereby the conventional DC to DCconverter has to be further improved to provide more stable DC powersupply.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a slope compensationcircuit for a peak current control mode power converter circuit. Thepeak current control mode power converter circuit has a first forwardconverter, a second forward converter and a pulse width modulation (PWM)controller for controlling the first forward converter and the secondforward converter. The first forward converter and the second forwardconverter respectively have a first high frequency transformer and asecond high frequency transformer and a first primary switch and asecond primary switch that are serial connected to a primary side of thefirst high frequency transformer and the second high frequencytransformer. The slope compensation circuit has a first chargingcircuit, a second charging circuit and a discharging circuit.

The first charging circuit has two resistors to be serial connected toform a first serial connected resistor and a capacitor. Two terminals ofthe first serial connected resistor are respectively coupled to a firstterminal of the capacitor and a first DC power source. A second terminalof the capacitor is coupled to multiple output voltage feedbackterminals of the PWM controller.

The second charging circuit has two resistors to be serial connected toform a second serial connected resistor. The second serial connectedresistor is parallel connected to the first serial connected resistor.

The discharging circuit has a third electronic switch and a fourthelectronic switch respectively coupled to multiple corresponding serialconnection nodes of the first serial connected resistor and the secondserial connected resistor. A control terminal of the third electronicswitch is coupled to a control terminal of the first primary switch ofthe first forward converter. A control terminal of the fourth electronicswitch is coupled to the control terminal of the second primary switchof the second forward converter. In this way, the third electronicswitch and the fourth electronic switch are simultaneously turned on oroff with the first primary switch and the second primary switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 2B are a detailed circuit diagram of a slope compensationcircuit for a peak current control mode power converter circuit inaccordance with the present invention;

FIG. 2A to FIG. 2C show equivalent circuit diagrams of a first preferredembodiment of a slope compensation circuit in accordance with thepresent invention;

FIGS. 3A to 3F show multiple voltage to current waveform diagrams ofinput and output terminals of a pulse width modulation (PWM) controllerin accordance with the present invention;

FIGS. 4A and 4B show two voltage waveform diagrams of the high leveltriangular wave DC voltage feedback signals V_(COMP1) and V_(COMP2)encounter corresponding voltage peak value V_(CS1) and V_(CS2) of thefirst high frequency transformer and the second high frequencytransformer in accordance with the present invention;

FIG. 5 is a second preferred embodiment of a triangular wave generatorin accordance with the present invention;

FIG. 6 is a third preferred embodiment of a triangular wave generator inaccordance with the present invention;

FIG. 7A is a fourth preferred embodiment of a triangular wave generatorin accordance with the present invention;

FIG. 7B is a fifth preferred embodiment of a triangular wave generatorin accordance with the present invention;

FIG. 8 is a circuit diagram of the conventional peak current controlmode power converter circuit in accordance with the prior art;

FIG. 9 is a current waveform diagram of current feedback signals of twocurrent feedback input terminals when two primary switches Q1 and Q2 aredriven by a pulse width signal of 50% duty cycle in accordance with theprior art;

FIG. 10 is a current waveform diagram of current feedback signals of twocurrent feedback input terminals when two primary switches Q1 and Q2 aredriven by a pulse width signal of 55% duty cycle in accordance with theprior art; and

FIG. 11 is a voltage waveform diagram of the power converter circuit ofFIG. 8 in accordance with the prior art.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1A and 1B, a first preferred embodiment inaccordance with the present invention of a slope compensation circuit 21a for a peak current control mode power converter circuit 10 is shown.In order to effectively avoid using high efficient and large sizedtransformers, the peak current control mode power converter circuit 10has parallel connected outputs of a first forward converter 11 and asecond forward converter 12, so as to provide only one output directcurrent (DC) power supply OUT. The first forward converter 11 and thesecond forward converter 12 are controlled by a pulse width modulation(PWM) controller 13.

The first forward converter 11 and the second forward converter 12respectively have a first high frequency transformer T1 and a secondhigh frequency transformer T2 and a first primary switch Q4 and a secondprimary switch Q6. Primary sides of the first high frequency transformerT1 and the second high frequency transformer T2 are coupled to an inputDC power supply. The first primary switch Q4 and the second primaryswitch Q6 are respectively serial connected to the primary sides of thefirst high frequency transformer T1 and the second high frequencytransformer T2. The first primary switch Q4 and the second primaryswitch Q6 are turned on or off to determine whether the first highfrequency transformer T1 or the second high frequency transformer T2 areconductive.

The PWM controller 13 has a first PWM output terminal OUT1 and a secondPWM output terminal OUT2, a first output voltage feedback terminal COMP1and a second output voltage feedback terminal COMP2, and a first currentfeedback input terminal CS1 and a second current feedback input terminalCS2. The two PWM output terminals OUT1 and OUT2 are respectively coupledto control terminals of the first primary switch Q4 and the secondprimary switch Q6 of the first forward converter 11 and the secondforward converter 12.

The two output voltage feedback terminals COMP1 and COMP2 arerespectively coupled to corresponding DC output terminals of the firstforward converter 11 and the second forward converter 12, so as toacquire two output voltage feedback signals. Moreover, the currentfeedback input terminals CS1 and CS2 respectively acquire conductivecurrent signals of the primary sides of the first high frequencytransformer T1 and the second high frequency transformer T2.

The slope compensation circuit 21 a is coupled to one DC voltage outputof the peak current control mode power converter circuit 10 and the twooutput voltage feedback terminals COMP1 and COMP2 of the PWM controller13. In this way, a high level triangular wave DC voltage feedback signaland the original DC voltage feedback signal are simultaneously inputtedto the two output voltage feedback terminals COMP1 and COMP2 of the PWMcontroller 13.

With reference to FIG. 2C, the first preferred embodiment of the slopecompensation circuit 21 a has a first charging circuit, a secondcharging circuit and a discharging circuit.

The first charging circuit has two resistors R63 and R66 and a capacitor24. The two resistors R63 and R66 are serial connected forming a firstserial connected resistor 22. Two terminals of the first serialconnected resistor 22 are respectively coupled to a first terminal ofthe capacitor 24 and a first DC power source +V1. A second terminal ofthe capacitor 24 is coupled to the output voltage feedback terminalsCOMP1 and COMP2 of the PWM controller 13.

The second charging circuit has two resistors R68 and R67 serialconnected forming a second serial connected resistor 23. The secondserial connected resistor 23 is parallel connected to the first serialconnected resistor 22.

The discharging circuit has a third electronic switch 25 and a fourthelectronic switch 26. The third electronic switch 25 and the fourthelectronic switch 26 are respectively coupled to corresponding serialconnection nodes of the first serial connected resistor 22 and thesecond serial connected resistor 23. A control terminal of the thirdelectronic switch 25 is coupled to the control terminal of the firstprimary switch Q4 of the first forward converter 11. A control terminalof the fourth electronic switch 26 is coupled to the control terminal ofthe second primary switch Q6 of the second forward converter 12. In thisway, the third electronic switch 25 and the fourth electronic switch 26are simultaneously turned on or off with the first primary switch Q4 andthe second primary switch Q6.

The following description explains the first preferred embodiment ofgenerating the high level triangular wave DC voltage feedback signal ofthe slope compensation circuit 21 a of a triangular wave generator.

With reference to FIG. 1, FIG. 2A, FIG. 2B and FIG. 2C, the outputterminal of the slope compensation circuit 21 a is coupled to the outputvoltage feedback terminals COMP1 and COMP2 of the PWM controller 13. Theoutput voltage feedback terminals COMP1 and COMP2 are coupled to asecond DC power source V2 via an internal resistor R_(IN). A voltage ofthe second DC power source V2 is lower than the voltage of the first DCpower source V1. Equivalent circuit diagrams are shown in FIG. 2A toFIG. 2C. An assumption in this preferred embodiment is to make aphototransistor of an optical coupler M4 as an electric current sourceI_(O).

With reference to FIGS. 3A to 3F, when the PWM controller 13 outputs apulse width signal to the first primary switch Q4 during a time periodt₁₀ to t₁₂, the first primary switch Q4 is conductive. At this moment,the first high frequency transformer T1 is inducted to have a conductivecurrent via a first current transformer T3. At this moment, the thirdelectronic switch 25 is also conductive. The second primary switch Q6and the fourth electronic switch 26 are in an off status. Since thefirst DC power source V1 is higher than the second DC power source V2and also the third electronic switch 25 makes the serial connectionnodes of the first charging circuit short, the first DC power source V1charges the capacitor 24 via the second charging circuit and alsoV_(COMP1) and V_(COMP2) ramp up as shown in FIGS. 3E and 3F.

With reference to FIG. 2B, when the PWM controller 13 controls thesecond primary switch Q6 to be conductive during a time period t₁₁ tot₁₂, the third electronic switch 25 and the fourth electronic switch 26are simultaneously conductive. At this moment, the fully chargedcapacitor 24 starts to discharge to the ground via the third electronicswitch 25 and the fourth electronic switch 26 and also V_(COMP1) andV_(COMP2) drop down. With reference to FIG. 4A, when the capacitor 24discharges for a period of time, the V_(COMP1) encounters acorresponding voltage peak value V_(CS1) of the first current feedbacksignal of the first high frequency transformer T1. At this moment, thePWM controller 13 immediately controls the first primary switch Q4 andthe third electronic switch 25 to be tuned off. With reference to FIG.2C and FIGS. 3A to 3F, the PWM controller 13 only continues to controlthe second primary switch Q6 and the fourth electronic switch 26 to beconductive at this moment and also the capacitor 24 charges via thefirst charging circuit. The PWM controller 13 then further controls thefirst primary switch Q4 and the third electronic switch 25 to beconductive, so that the capacitor 24 starts to discharge. And then thetriangular wave DC voltage feedback signals, which are V_(COMP1) andV_(COMP2), drop down. When the corresponding voltage peak value V_(CS2)of the second current feedback signal of the second high frequencytransformer T2 is encountered, the PWM controller 13 controls the secondprimary switch Q6 and the fourth electronic switch 26 to be turned off,so as to operate in a cycle.

With reference to FIG. 4A and FIG. 4B, the slope compensation circuit 21a of the present invention indeed compensates a slope of the output DCvoltage of the power converter circuit to make the original outputvoltage feedback signal compensate as the high level triangular wave DCvoltage feedback signals V_(COMP1) and V_(COMP2). In this way, theconventional voltage surge that would turn off the first primary switchQ4 and the second primary switch Q6 ahead of the pre-determined timeproduced by the conductive high frequency transformer can be avoided.Hence the first primary switch Q4 and the second primary switch Q6 canbe turned off on time when the time point t₁₂ of the target valuereaches.

A raised slop and a dropped slop of the high level triangular wave DCvoltage feedback signals can be changed along with a resistor value anda capacitor value of the first charging circuit and the second chargingcircuit. In this preferred embodiment, the resistor R63 is equal to theresistor R68 and the resistor R66 is equal to the resistor R67 to makethe two current circuits have the same charge and discharge property.

With reference to FIG. 5, a second preferred embodiment of the slopecompensation circuit 21 b of the present invention is similar to thefirst preferred embodiment. The only difference is that the first serialconnected resistor R66 and the second serial connected resistor R67coupled to the capacitor 24 are respectively parallel connected to afirst diode D1 and a second diode D2 to adjust RC charge and dischargetime.

With reference to FIG. 6, a third preferred embodiment of the slopecompensation circuit 21 c in accordance with the present invention issimilar to the first preferred embodiment. In this preferred embodiment,the control terminals of the third electronic switch 25 and the fourthelectronic switch 26 are coupled to the primary sides of the first highfrequency transformer T1 and the second high frequency transformer T2via a first current transformer 28 and a second current transformer 29that are respectively coupled to the primary sides of the high frequencytransformers T1 and T2, so as to acquire conductive current signals ofthe primary sides of the high frequency transformers T1 and T2. Theconductive current signals are synchronous as driven signals of enablingthe first primary switch Q4 and the second primary switch Q6, so as tobe a basis of disabling the third electronic switch 25 and the fourthelectronic switch 26.

With reference to FIG. 7A, a fourth preferred embodiment of the slopecompensation circuit 21 d of the present invention is similar to thefirst preferred embodiment. The only difference is that the thirdelectronic switch 25 and the fourth electronic switch 26 are replaced bya first diode 30 and a second diode 31. A positive electrode of thefirst diode 30 is coupled to the serial connection node of the secondserial connected resistor 23 of the second charging circuit and anegative electrode of the first diode 30 is coupled to the controlterminal of the second primary switch Q6. Similarly, a positiveelectrode of the second diode 31 is coupled to the serial connectionnode of the first serial connected resistor 22 of the first chargingcircuit and a negative electrode of the second diode 31 is coupled tothe control terminal of the first primary switch Q4.

With reference to FIG. 7B, a fifth preferred embodiment of the slopecompensation circuit 21 d of the present invention is similar to thefourth preferred embodiment. The only difference is that a negativeelectrode of the first diode 30 is coupled to the second currenttransformer 29 coupled to the primary side of the second high frequencytransformer T2. Similarly, a negative electrode of the second diode 31is coupled to the first high frequency transformer T1.

The aforesaid electronic switches can be transistors of MOSFET or BJT,so the control terminal is a gate of the MOSFET or a base of the BJT.

It can be understood from the above multiple preferred embodiments thatthe slope compensation method in accordance with the present inventionis as follows.

Firstly, a first and second driving signals respectively synchronized toa switch driving signals of enabling the first primary switch and thesecond primary switch are acquired. And then two triangular wave signalsare generated. The first and second driving signals of the first primaryswitch and the second primary switch determines a ramp up times of thecorresponding triangular wave signals.

Thirdly, the two triangular wave signals are added to one of the outputDC voltage feedback signal of the corresponding power converter circuitthat are used to compare with the current peak value of the voltagefeedback signals. Hence the triangular wave signal forms the high leveltriangular wave DC voltage feedback signal that is higher than the DCvoltage feedback signal. The triangular wave DC voltage feedback signalis inputted to the output voltage feedback signal of the PWM controller.And thereby the PWM controller compares a peak value of the triangularwave DC voltage feedback signal and a corresponding voltage value of theprimary side current signal of the first and the second high frequencytransformers, so as to generate the first and second switch drivingsignals of the first and second primary switches.

To sum up, the present invention compensates the slope of the output DCvoltage feedback signal to effectively avoid the surge of conductivecurrent signals when the high level triangular wave DC voltage feedbacksignal compares with successive conductive current signals. Hence theelectronic switches of the high frequency transformers are not turnedoff ahead of the pre determined time due to the surge. Therefore theaforesaid slope compensation method can effectively reduce noises of thealternately conducted electronic switches and also can improve thedrawback of the unstable output DC power oscillation.

While the invention has been described by way of example and in terms ofa preferred embodiment, it is to be understood that the invention is notlimited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

1. A slope compensation circuit for a peak current control mode powerconverter circuit, wherein the peak current control mode power convertercircuit has a first forward converter, a second forward converter and apulse width modulation (PWM) controller for controlling the first andsecond forward converters, wherein the first forward converter has afirst high frequency transformer and a first primary switch serialconnected to a primary side of the first high frequency transformer; andthe second forward converter has a second high frequency transformer anda second primary switch serial connected to a primary side of the secondhigh frequency transformer, wherein the slope compensation circuitcomprises: a first charging circuit having two resistors to be serialconnected to form a first serial connected resistor and a capacitor,wherein two terminals of the first serial connected resistor arerespectively coupled to a first terminal of the capacitor and a first DCpower source, wherein a second terminal of the capacitor is coupled to aplurality of output voltage feedback terminals of the PWM controller; asecond charging circuit having two resistors to be serial connected toform a second serial connected resistor, wherein the second serialconnected resistor is parallel connected to the first serial connectedresistor; and a discharging circuit having a third electronic switchcoupled to a serial connection node of the first serial connectedresistor and having a control terminal coupled to a first driving signalsynchronized to a switch driving signal of a control terminal of thefirst primary switch; and a fourth electronic switch coupled to a serialconnection node of the second serial connected resistor and having acontrol terminal coupled to a second driving signal synchronized to aswitch driving signal of a control terminal of the second primaryswitch, whereby the third electronic switch and the fourth electronicswitch simultaneously turned on or off with the first primary switch andthe second primary switch.
 2. The slope compensation circuit as claimedin claim 1, wherein the control terminal of third electronic switch iscoupled to the control terminal of the first primary switch; and thecontrol terminal of the fourth electronic switch is coupled to thecontrol terminal of the second primary switch.
 3. The slope compensationcircuit as claimed in claim 1, wherein the control terminal of thirdelectronic switch is coupled to the primary side of the first highfrequency transformer; and the control terminal of the fourth electronicswitch is coupled to the primary side of the second high frequencytransformer.
 4. The slope compensation circuit as claimed in claim 1,wherein the two resistors of the first serial connected resistor arecorresponding equal to the two resistors of the second serial connectedresistor.
 5. The slope compensation circuit as claimed in claim 2,wherein the two resistors of the first serial connected resistor arecorresponding equal to the two resistors of the second serial connectedresistor.
 6. The slope compensation circuit as claimed in claim 3,wherein the two resistors of the first serial connected resistor arecorresponding equal to the two resistors of the second serial connectedresistor.
 7. The slope compensation circuit as claimed in claim 1,wherein the resistors of the first serial connected resistor and thesecond serial connected resistor coupled to the capacitor are furtherparallel connected to a diode.
 8. The slope compensation circuit asclaimed in claim 2, wherein the resistors of the first serial connectedresistor and the second serial connected resistor coupled to thecapacitor are further parallel connected to a diode.
 9. The slopecompensation circuit as claimed in claim 3, wherein the resistors of thefirst serial connected resistor and the second serial connected resistorcoupled to the capacitor are further parallel connected to a diode. 10.The slope compensation circuit as claimed in claim 1, wherein the secondterminal of the capacitor is further coupled to one of multiple outputDC voltage terminals of the power converter circuit via an opticalcoupler, wherein an light emitting diode (LED) terminal is coupled tothe output DC voltage terminal and a phototransistor is coupled to thecapacitor.
 11. The slope compensation circuit as claimed in claim 2,wherein the second terminal of the capacitor is further coupled to oneof multiple output DC voltage terminals of the power converter circuitvia an optical coupler, wherein an light emitting diode (LED) terminalis coupled to the output DC voltage terminal and a phototransistor iscoupled to the capacitor.
 12. The slope compensation circuit as claimedin claim 3, wherein the second terminal of the capacitor is furthercoupled to one of multiple output DC voltage terminals of the powerconverter circuit via an optical coupler, wherein an light emittingdiode (LED) terminal is coupled to the output DC voltage terminal and aphototransistor is coupled to the capacitor.
 13. A slope compensationcircuit for a peak current control mode power converter circuit, whereinthe peak current control mode power converter circuit has a firstforward converter, a second forward converter and a pulse widthmodulation (PWM) controller for controlling the first and second forwardconverters, wherein the first forward converter has a first highfrequency transformer and a first primary switch serial connected to aprimary side of the first high frequency transformer; and the secondforward converter has a second high frequency transformer and a secondprimary switch serial connected to a primary side of the second highfrequency transformer, wherein the slope compensation circuit comprises:a first charging circuit having two resistors to be serial connected toform a first serial connected resistor and a capacitor, wherein twoterminals of the first serial connected resistor are respectivelycoupled to a first terminal of the capacitor and a first DC powersource, wherein a second terminal of the capacitor is coupled to one ofa plurality of output voltage feedback terminals of the PWM controller;a second charging circuit having two resistors to be serial connected toform a second serial connected resistor, wherein the second serialconnected resistor is parallel connected to the first serial connectedresistor; and a discharging circuit having a first diode having apositive electrode coupled to a serial connection node of the secondserial connected resistor; and a negative electrode coupled to a firstdriving signal synchronized to a switch driving signal of a controlterminal of the second primary switch; a second diode having a positiveelectrode coupled to a serial connection node of the first serialconnected resistor; and a negative electrode coupled to a second drivingsignal synchronized to a switch driving signal of a control terminal ofthe first primary switch.
 14. The slope compensation circuit as claimedin claim 13, wherein the negative electrode of the first diode iscoupled to the control terminal of the second primary switch; and thenegative electrode of the second diode is coupled to the controlterminal of the first primary switch.
 15. The slope compensation circuitas claimed in claim 13, wherein the negative electrode of the firstdiode is coupled to a primary side of a second high frequencytransformer; and the negative electrode of the second diode is coupledto a primary side of a first high frequency transformer.
 16. The slopecompensation circuit as claimed in claim 13, wherein the two resistorsof the first serial connected resistor are corresponding equal to thetwo resistors of the second serial connected resistor.
 17. The slopecompensation circuit as claimed in claim 14, wherein the two resistorsof the first serial connected resistor are corresponding equal to thetwo resistors of the second serial connected resistor.
 18. The slopecompensation circuit as claimed in claim 15, wherein the two resistorsof the first serial connected resistor are corresponding equal to thetwo resistors of the second serial connected resistor.
 19. The slopecompensation circuit as claimed in claim 13, wherein the resistors ofthe first serial connected resistor and the second serial connectedresistor coupled to the capacitor are further parallel connected to adiode.
 20. The slope compensation circuit as claimed in claim 14,wherein the resistors of the first serial connected resistor and thesecond serial connected resistor coupled to the capacitor are furtherparallel connected to a diode.
 21. The slope compensation circuit asclaimed in claim 15, wherein the resistors of the first serial connectedresistor and the second serial connected resistor coupled to thecapacitor are further parallel connected to a diode.
 22. The slopecompensation circuit as claimed in claim 13, wherein the second terminalof the capacitor is further coupled to one of multiple output DC voltageterminals of the power converter circuit via an optical coupler, whereinan light emitting diode (LED) terminal is coupled to an output DCvoltage terminal and a phototransistor is coupled to the capacitor. 23.The slope compensation circuit as claimed in claim 14, wherein thesecond terminal of the capacitor is further coupled to one of multipleoutput DC voltage terminals of the power converter circuit via anoptical coupler, wherein an light emitting diode (LED) terminal iscoupled to an output DC voltage terminal and a phototransistor iscoupled to the capacitor.
 24. The slope compensation circuit as claimedin claim 15, wherein the second terminal of the capacitor is furthercoupled to one of multiple output DC voltage terminals of the powerconverter circuit via an optical coupler, wherein an light emittingdiode (LED) terminal is coupled to an output DC voltage terminal and aphototransistor is coupled to the capacitor.
 25. A slope compensationmethod for a peak current control mode power converter circuit whereinthe peak current control mode power converter circuit has a firstforward converter, a second forward converter and a pulse widthmodulation (PWM) controller for controlling the first and second forwardconverters, wherein the first forward converter has a first highfrequency transformer and a first primary switch serial connected to aprimary side of the first high frequency transformer; and the secondforward converter has a second high frequency transformer and a secondprimary switch serial connected to a primary side of the second highfrequency transformer; wherein the slope compensation method comprises:acquiring a first and second driving signal respectively synchronized toa first and second switch driving signals of enabling the first primaryswitch and the second primary switch; generating two triangular wavesignal wherein the first and second switch driving signals of the firstprimary switch and the second primary switch determines a ramp up timesof the corresponding triangular wave signals; and adding the twotriangular wave signals to one of the output DC voltage feedback signalof the corresponding power converter circuit that are used to comparewith a current peak value of the voltage feedback signals, so as to forma high level triangular wave DC voltage feedback signal that is higherthan the DC voltage feedback signal, wherein the triangular wave DCvoltage feedback signal is inputted to the output voltage feedbacksignal of the PWM controller, wherein the PWM controller compares thepeak value of the triangular wave DC voltage feedback signal and acorresponding voltage value of the primary side current signal of thefirst and the second high frequency transformers, so as to generate thefirst and second switch driving signals of the first and second primaryswitches.
 26. The slope compensation method as claimed in claim 25,wherein the synchronous signal is acquired from a driven terminal of thefirst primary switch and the second primary switch.
 27. The slopecompensation method as claimed in claim 25, wherein the synchronoussignal is acquired from a driven signal of the first primary switch andthe second primary switch, which is generated from the currenttransformer coupled to the primary side of the two high frequencytransformers.